1. Field of the Invention
The present invention relates to a rolling bearing, and particularly relates to a rolling bearing for use, for example, in an engine accessory (alternator, compressor, water pump, etc.) for a transmission or a continuously variable transmission (toroidal CVT or belt CVT) in a car, an agricultural machine, a construction machine, a steel machine, or the like under the environment where foreign matters such as metal burrs or mud are mixed so that the lubricating condition is apt to deteriorate.
2. Description of the Related Art
In the related art, high-carbon chromium bearing steel prescribed in JIS, particularly SUJ2 is generally used as material for the rolling bearings. Such material to be used is subjected to hardening and tempering treatment so that the material is used in the condition that the surface hardness HRC (Rockwell hardness) is about 62 and the residual austenite quantity is about 10 wt %.
In such a related art rolling bearing, however, if foreign matters are mixed into the lubricating oil of the bearing, the rolling life thereof is conspicuously shortened in comparison with the case where clean lubricating oil is used. In the lubricating oil, metal cuttings, shavings, burrs, scourings, muddy water, and so on, are mixed. In such an environment of use of a rolling bearing in which such foreign matters are mixed, the foreign matters cause a dent (damage) or rust on the orbital surfaces of races and rolling elements of the rolling bearing, and flaking due to the dent or rust so that the life of the rolling bearing is reduced extremely.
It is noted that the life of the rolling bearing is reduced to about xe2x85x9 as long as that in the case where clean lubricating oil is used in accordance with the quantity, hardness and size of the foreign matters, as described in xe2x80x9cForeign-Matter Mixing Conditions and Rolling Fatigue Lifexe2x80x9d (NSK Technical Journal No. 655, pp. 17-24, 1993) as experimental researches about such circumstances. This means there is reproduced such a phenomenon that the orbital surface of a bearing is affected by the invasion of foreign matters and the rolling fatigue so that very small dents I a range from the order of xcexcm to the order of one hundred xcexcm or more are produced in the orbital surface, and flaking is developed, due to the dents, to lower the rolling life, like pitching which is seen in transmission gears in a car.
As a measure to prolong the life when such foreign matters are mixed, conventional research has sought to enhance the surface hardness of the bearing.
There is a related art example as such a measure (xe2x80x9cMetal Handbookxe2x80x9d, edited by The Japan Institute of Metals, revised third edition, pp. 780-797) in which a bearing is manufactured by use of precipitation-hardened tool steel (SKH or SKD) in which an oxide forming element is added so that plenty of carbide is precipitated. (Hereinafter, this related art example is referred to as xe2x80x9crelated art 1xe2x80x9d)
There are other measures to prolong the life, that is, the technique of Japanese Patent Examined Publication No. Hei.6-11899 (hereinafter referred to as xe2x80x9crelated art 2xe2x80x9d), the technique of Japanese Patent Unexamined Publication No. Hei.3-173747 (hereinafter referred to as xe2x80x9crelated art 3xe2x80x9d), and the technique of Japanese Patent Examined Publication No. Hei.7-110988 (hereinafter referred to as xe2x80x9crelated art 4xe2x80x9d).
According to the related art 2, material consisting of 0.4 to 0.8 wt % of C, 4.0 to 8.0 wt % of Cr, 0.3 to 1.2 wt % of Si, 1.0 wt % or less of Mn, and the residue of Fe and unavoidable impurities, is subjected to carburizing or carbonitriding treatment so as to form high-chromium bearing steel which has a long life and an excellent rolling fatigue life characteristic even under the environment where foreign matters are mixed into the lubricant oil for the rolling bearing.
The related art 3 discloses a grease-charged bearing composed of high-carbon stainless steel containing Cr in a range of from 13 wt % to 18 wt % while at least a fixed race is of martensitic stainless steel.
Further, according to the related art 4, at least one of races and rolling elements is made from alloy steel containing at least C and Cr in a range of from 0.3 wt % to 0.6 wt % and in a range of from 3.0 wt % to 14 wt % respectively, and has a surface layer portion formed by carburizing or carbonitriding and further thermal hardening. In addition, the quantity of fine carbide in a surface layer portion of at least one of the races and the rolling elements is in a range of from 20 vol % to 50 vol %, and the quantity of retained austenite in the surface layer portion is in a range of from 10 vol % to 25 vol %.
In the related art 1, there is indeed an advantage that the hardness of the rolling surface becomes high enough so that the rolling surface is hardly dented by foreign matters in the lubricating oil. On the other hand, precipitated carbide becomes bulky in accordance with the content of alloy elements to form the carbide. Accordingly, there is a fear that stress is concentrated around the carbide, and flaking breaks out from the stress-concentrated portion, so that the life is shortened.
In addition, in the related art 2, expensive heat treatment such as carburizing or carbonitriding treating is required so that the cost increases. In addition, since Cr is contained only in a range of from 4.0 wt % to 8.0 wt %, a passive film only about several xcexcm thick is formed on the race surface when muddy water is mixed. As a result, it may be considered that the passive film is broken by the dent due to pressure of foreign matters, or rust is generated to cause pitting. Thus, a satisfactory effect cannot be expected.
In addition, in the related art 3, 13Cr-0.7% C steel SUS440A, 18Cr-1% C steel SUS440C, or 13Cr-0.3% C steel SUS420J2, each having a tempered martensitic structure, is used as the martensitic stainless steel so that the resistance to hydrogen embrittlement is enhanced by a passive film. However, SUS420J2 having the surface hardness HRC of 52 is not sufficient for rolling fatigue. In addition, SUS440A and SUS420J2 do not have additive V or Mo for forming fine carbide, and it is inevitable that bulky eutectic carbide the size of which is larger than 10 xcexcm is apt to be precipitated on the rolling surface. In addition, there are not only a problem that flaking is produced in the rolling surface, but also a problem that the passive film becomes easy to be broken so that the resistance to corrosion is lowered and further the workability in manufacturing bearings is also deteriorated.
Further, in the related art 4, since expensive heat treatment such as carburizing or carbionitriding is required, further improvement is desired.
The present invention was developed in consideration of foregoing circumstances. It is an object of the present invention to provide a rolling bearing in which a firm passive film is formed on a race so as to well prevent flaking in early stages and prolong the bearing life on a large scale even under a poor lubricating environment where foreign matters or muddy water is mixed.
In order to achieve the above object, there is provided a rolling bearing including races and rolling elements, at least the race including: 0.60 wt % to 0.95 wt % of C; 10.0 wt % to 13.0 wt % of Cr; and at least one kind of 0.5 wt % to 2.0 wt % of Mo and 0.5 wt % to 2.0 wt % of V with the balance or Fe. The surface hardness of a raceway surface of the race is not less than HRC 58, and the raceway surface of the race has a passive firm with the thickness of 5 to 100 nm.
In addition, fine Mo or V carbide the size of which is in a range of from 30 nm to 300 nm s dispersed and precipitated at least on the raceway surface of the races.
Moreover, eutectic carbide the size of which is larger than 10 xcexcm is restrained from being produced at least on the raceway surface of the races in heat treatment.
The inventors of this application made various investigations about rolling bearings which were resistant to foreign matters, resistant to water and long in life in the state where stress was applied to the rolling bearings. As a result, the inventors obtained various knowledge about the relationship among the contents of respective elements and each of surface hardness, corrosion resistance, temper-softening resistance and passive film of a race, and attained the invention on the basis of this knowledge.
Description will be made about the effects of contained elements used in the present invention, the critical significance of their contents, and so on.
C (carbon) is an element for giving surface hardness required in a rolling bearing. 0.6 wt % or more of C is indispensable to obtain the surface hardness HRC of 58 or more. On the other hand, if the content of C is larger than 0.95 wt %, the matrix is made into martensite so that the hardness after hardening and tempering is improved. From the point of view of corrosion resistance, however, the smaller the content of C is, the better. This is because Cr forms bulky eutectic carbide in manufacturing steel if a large quantity of C is added. As a result, the Cr density is insufficient in the matrix so that enough corrosion resistance cannot be obtained, and the rolling life or the toughness is reduced. Particularly, it is preferable that the content of C is in a range of from 0.60 wt % to 0.95 wt %.
Cr (chromium) is the most effective element to give corrosion resistance to steel. If 10.0 wt % or more of Cr is contained, good corrosion resistance is obtained and a passive film is formed to be 5 nm or more thick. On the other hand, if the content of Cr is larger than 13 wt %, the corrosion resistance is further enhanced. However, if Cr is added beyond its necessity, xcex4-ferrite is produced so as to embrittle the steel easily. As a result, the toughness is reduced, and the workability is lowered conspicuously. In order to form a firm passive film the thickness of which is in a range of from 5 nm to 100 nm, it is sufficient to set the upper limit of the content of Cr to be 13 wt %. It is therefore preferable that the content of Cr is in a range of from 10.0 wt % to 13.0 wt %.
Mo (molybdenum) is an element having an effect to increase hardenability and temper-softening resistance conspicuously, having an effect to delay structural deformation caused by rolling fatigue, and further enhancing corrosion resistance. If the content of Mo is smaller than 0.5 wt %, the effects are not sufficient. In order to disperse fine Mo carbide the size of which is in a range of from 50 nm to 300 nm, 0.5 wt % or more of Mo is required. However, if excessive amount of Mo is added, the toughness and the workability are lowered. It is therefore preferable that the upper limit of the content of Mo is set to be 2.0 wt %, and the content of Mo is in a range of from 0.5 wt % to 2.0 wt %.
V (vanadium) is an element for forming fine carbide and nitride. V has not only an effect to restrain the formation of Cr carbide and nitride but also an effect to enhance hardness due to secondary precipitation in a tempering process at 400 to 550xc2x0 C. so as to have an effect to enhance strength conspicuously. In addition, 0.5 wt % or more of V is required for dispersing and precipitating fine V carbide the size of which is in a range of 50 nm to 300 nm so that bulky carbide having the size of 10 xcexcm or more is restrained from being produced. In consideration of the cost and the workability, it is preferable that the upper limit, of the content of V is set to be 2.0 wt %, and the content of V is in a range of from 0.5 wt % to 2.0 wt %.
On the other hand, Si (silicon) is an element to improve the delay in structural deformation, the hardenability and the temper-softening resistance. If the content of Si is smaller than 0.2 wt %, the effect is not sufficient. If the content of Si is larger than 1.5 wt %, the workability deteriorates conspicuously. Particularly, it is therefore preferable that the content of Si is in a range of from 0.2 wt % to 1.5 wt %.
Mn (manganese) is an element required as deoxidizer in steel making. 0.2 wt % or more of Mn is necessary. But if a large quantity of Mn is added, forgeability and machinability deteriorate conspicuously, and Mn coexists with impurities such as S, P, and so on, so as to lower corrosion resistance. It is therefore preferable that the upper limit of the content of Mn is set to be 1.0 wt %, and the content of Mn is in a range of from 0.2 wt % to 1.0 wt %.
Further, it is preferable that the content of O (oxygen) is not larger than 10 ppm because oxide enclosures which reduce the rolling life are restrained from beige produced. The con en of each of S (sulfur) and P (phosphorus) is preferably in a range of not larger than 0.02 wt %.
Here, for example, when a race, is heated and hardened at 1,000 to 1,200xc2x0 C. and then tempered at a high temperature (400 to 550xc2x0 C.), fine Mo/V carbide is precipitated on the race and bulky eutectic carbide is restrained from being produced so that a firm passive film is formed. However, even if the race is tempered at a low temperature (180 to 220xc2x0 C.), bulky eutectic carbide is not produced, and a firm passive film is formed.
On the other hand, if bulky eutectic carbide the size of which is 10 xcexcm or more is produced on the race, portions having a passive film formed therein and portions having no passive film are produced. As a result, flaking due to corrosive pitting or due to the eutectic carbide is apt to arise. However, if eutectic carbide the size of which is larger than 10 xcexcm is restrained from being produced on the race in heat treatment as in the present invention, a passive film is formed uniformly over the race so that the life is prolonged.